Platinum drugs are the most active agents in ovarian cancer. Their cytotoxicty results from DNA crosslinking. High tumour response rates are seen, but 80 % of patients relapse. Major mechanisms of platinum resistance in patients remain to be established. We have studied DNA interstrand crosslinking and its repair in response to ex vivo treatment with cisplatin in forty patients with ovarian cancer using the single cell gel electrophoresis (comet) assay. Tumour cells from resected tumours or tumour and mesothelial cells from ascites were obtained from chemonaive patients and those relapsing after platinum-based therapy. The average percent decrease in tail moment at the peak of crosslinking was 61.1% 9.25 in 34 pre-chemotherapy patient samples following treatment with lOOuM cisplatin. In 14 post-chemotherapy patient samples it was 58.1% 9.94. The average percentage repair at 24 hours was 3.6% 18.89 in pre- chemotherapy patients and 44.6% 43.4 for post-chemotherapy patients (p < 0.001). In 6 paired samples, before and after chemotherapy the average percentage repair at 24 hours was 7.2% 12.64 increasing to 69.5% 23.42 after chemotherapy. Differences in cell cycling, and cell signalling gene expression levels using microarray analysis was found, between pre- and post-chemotherapy patients. Real time PCR was also used to investigate the levels of ERCC1 (excision-related cross complementation group 1) in 3 of these paired patient samples, which was found to be increased by an average of 14.4% +/-0.8% in 3 post-chemotherapy samples. In ten pre-chemotherapy and seven post-chemotherapy patient tumours incubated ex vivo with 50uM melphalan, the percent decrease in tail moment at the peak of crosslinking was 41.4+11.2, and 44.6 7.6, respectively. 24 hours later the percentage repair was 3.1 .25.6 for untreated and 2.8 26.3 for treated tumours. In conclusion, repair of DNA interstrand crosslinks appears to be an important mechanism of clinical platinum resistance in ovarian cancer. Repair of melphalan crosslinks is unaffected.